1. Field of the Invention
The present invention relates to a method for determining depth of focus when forming a resist pattern in a substrate utilizing photolithography.
More specifically, the present invention relates to a method for determining depth of focus when forming a resist pattern in a substrate surface of a semiconductor substrate or the like, which is formed of silicon or the like, by exposing the resist by reduction-projection of a reticle mask and developing the resist.
2. Description of the Related Art
Fabrication of semiconductor circuits has included a process in which a reticle mask, on a surface of which a circuit pattern is formed, is disposed over a semiconductor substrate (which may be referred to as a “wafer” hereafter). The circuit pattern is reduction-projected onto a resist layer formed on a surface of the wafer, using a reduction-projection exposure apparatus, a scanning type reduction-projection exposure apparatus or the like (referred to as an “exposure apparatus” below). The exposure process described above is repeated a number of times, until the semiconductor circuit is completed.
There have been continuing demands for a reduction of scale of a resist pattern that is formed on the surface of the wafer after exposure and development. A focusing position from which ultraviolet light is irradiated perpendicularly onto the wafer during exposure (below referred to as “depth of focus”) is one of factors that determine whether the quality of the resist pattern that is formed is satisfactory or not. It is necessary to control upper/lower limits of the depth of focus during exposure, such that satisfactory resist patterns can be formed consistently. These upper/lower limits of depth of focus are assured by observing the resist pattern with a critical dimension measurement SEM from a perpendicular direction relative to the wafer (i.e., from above the wafer), measuring and evaluating finished dimensions (principally, a bottom dimension), and further, judging the form of the resist pattern that is being observed by visual inspection.
(Conventional Depth of Focus Determination Method)
A depth of focus determination method according to such a conventional method is concretely described below.
First, a resist layer is formed at a surface of a wafer, with the same reticle mask being used for each time of exposure at different exposure locations. At this time, exposure is performed with the depth of focus changing when the exposure location changes, and other exposure conditions being constant. Thereafter, the resist layer is developed as a whole, and thus a plurality of resist patterns for the respectively different depths of focus are formed on the wafer surface.
FIG. 6 is a schematic diagram showing an example of resist patterns formed on a wafer surface under exposure conditions with different depths of focus. FIG. 6 shows a plurality of sections into which the surface of a wafer 1 is divided in a checkered pattern. The sections are provided with resist patterns formed by exposing, by reduction-projection with a reticle mask, and developing the sections. The numerical values given in the sections represent depths of focus during exposure.
Evaluation of the resist pattern formed as described above is implemented by observing band-like resist patterns, which are formed in correspondence to a portion at the surface of the reticle mask, which portion is formed with a band-like pattern having a certain line width.
FIGS. 7A and 7B are schematic diagrams of the aforementioned band-like resist pattern. FIG. 7A shows a plan view of the band-like resist pattern, and FIG. 7B shows a sectional view, in a line width direction, of the band-like resist pattern. In FIGS. 7A and 7B, the reference numeral 1 represents the wafer and the reference numeral 2 represents the band-like resist pattern. The arrow B represents a bottom dimension and the arrow T represents a top dimension.
Evaluation of the band-like resist pattern is performed by judging a shape thereof. This judgement is performed by, at the same time as measurement of the bottom dimension represented by the arrow B in FIGS. 7A and 7B, which corresponds substantially to a dimension of the pattern formed on the surface of the wafer, obtaining a plan view of the band-like resist pattern as an image, as shown in FIG. 7A, and observing this image visually. This evaluation is further performed for each of the band-like resist patterns formed by exposing and developing at the respective depths of focus. The upper/lower limits of the depths of focus are judged by a procedure which is described below.
First, the bottom dimensions of the band-like resist patterns formed at the respective depths of focus are measured. FIG. 8 is a graph of these bottom dimensions.
FIG. 8 shows an example of variation of the bottom dimension of the band-like resist patterns with respect to depth of focus. When the band-like resist patterns used for measurement of the bottom dimensions shown in FIG. 8 are measured, exposure conditions other than the depth of focus are fixed such that a maximum value of the bottom dimension of the pattern for a depth of focus of 0 μm is 0.2 μm. As can be seen in FIG. 8, the bottom dimension varies along a parabolic curve, becoming greater with an increase in depth of focus, and then, beyond the maximum dimension (0.2 μm) exhibited for a depth of focus of 0 μm, becoming smaller in accordance with the curve.
Here, “depth of focus” refers to a focusing position of ultraviolet light that is irradiated during exposure, with respect to a perpendicular direction of the wafer. A positive value represents a focusing position further from a light source (exposure apparatus), and a negative value represents a focusing position closer to the light source. Moreover, a depth of focus of 0 μm (a reference value of the depth of focus) corresponds to the depth of focus for which the bottom dimension is at the maximum value with respect to depth of focus.
There are variations in the dimensions depending upon location in the wafer when the resist pattern is formed, even given the same exposure conditions, and it is necessary that these variations of the dimensions be constrained to within a certain range. This range of the variations of the dimensions must be determined such that, at least, the customers' minimum product quality requirements can be achieved, while giving due consideration to production costs of the semiconductor circuit and yields. In view of this point, the range of variation is generally of the order of ±10% relative to target dimensions.
In FIG. 8, the allowable values of the range of variations are ±10% relative to the target dimension, and the allowable range for the bottom dimension is 0.18 μm to 0.22 μm. Focusing depths corresponding to this range are within a range shown by a broken line L and a broken line H, that is, a range from −0.4 μm to +0.4 μm. In other words, a lower limit of the depth of focus is −0.4 μm and an upper limit is +0.4 μm.
However, as the depth of focus is increased, effects such as a remarkable decrease in resist layer thickness (below referred to as “film thickness reduction”) and film peeling of a resist pattern surface layer portion occur in resist patterns exposed at a depth of focus greater than a certain value. Such film thickness reduction and film peeling start to be especially noticeable at a focus depth with a value such as that shown by a broken line H′ in FIG. 8, and occur extremely often for values that are only slightly smaller than an upper limit value of the depth of focus found as described above. Consequently, an upper limit of the depth of focus that is allowable in practice would be 0.3 μm, as shown by the broken line H′ in FIG. 8. In the following descriptions of the conventional art and the present invention, “upper limit value” means a value at or above which film thickness reduction and film peeling are especially noticeable.
Determination of such an upper limit value is performed by obtaining and visually observing an image of the form of the band-like resist pattern (a form corresponding to the plan view shown in FIG. 7A) using a critical dimension measurement SEM.
Specifically, the upper limit value is determined by implementing the following steps (1) and (2) in this order.
(1) Confirming a depth of focus at which, as the depth of focus increases, film peeling begins to occur.
(2) Comparatively judging various states, which are attributable to film thickness reduction, of the form of the band-like resist pattern for depths of focus around the depth of focus at which film peeling occurred.
For the comparative judgement of step (2), a relationship can be utilized in which, as the depth of focus increases, a difference between the top dimension and the bottom dimension becomes smaller as the occurrence of film thickness reduction becomes remarkable, as shown in FIG. 9.
FIG. 9 is a schematic diagram showing changes in the form of the band-like resist pattern with respect to changes in the depth of focus, as measured with bottom dimensions corresponding to the depths of focus shown in FIG. 8.
In FIG. 9, the values in the upper boxes are the depths of focus, the middle boxes are plan views showing the various forms of the band-like resist pattern for these depths of focus, and the lower boxes are sectional views showing the various forms of the band-like resist pattern for these depths of focus. The forms and relative dimensions of the band-like resist patterns shown in the middle and lower boxes are not intended to accurately reflect the forms and relative dimensions in actuality, but show general trends for the purposes of explanation.
As can be seen from FIG. 9, the resist layer thickness decreases remarkably at depths of focus of 0.3 μm and greater. In correspondence with this decrease, the difference between the top dimension and the bottom dimension diminishes rapidly.
(Problems With The Method of the Prior Art)
However, in the method for determining the upper limit of depth of focus of the prior art, because the band-like resist pattern is observed in a perpendicular direction of the wafer, as shown in the plan view of FIG. 7A, there are cases in which it is difficult to observe film peeling, which is attributable to peeling of the resist, whose principal component is in the wafer perpendicular direction. Moreover, any judgement in accordance with the above-described steps (1) and (2) is a sensory evaluation based on visual observation, and thus evaluation will be imprecise. Accordingly, an upper limit value of 0.3 μm determined as described above means “around about 0.3 μm” in practice, and accuracy is insufficient. That is, in practice the upper limit value represented by the broken line H′ in FIG. 8 has a certain amount of play in the horizontal axis direction. This play means that there are inconsistencies attributable to the visual evaluation.
Such inconsistencies are substantially attributable to impreciseness resulting from the sensory evaluation, as shown in the following items (1) and (2). Further, although there is a way to deal with this impreciseness, as shown in item (3) below, it may be difficult to provide satisfactory accuracy because the evaluation is essentially a sensory evaluation. Moreover, in order to improve the accuracy in this way it is necessary to perform measurement and evaluation at many points, which is also disadvantageous.
(1) Because the observation of the band-like resist pattern is visual, accurate judgement of the form is difficult.
(2) There are inconsistencies in judgement attributable to operators who are visually judging the form of the band-like resist pattern.
(3) It is difficult to judge the form by observing only one location of the band-like resist pattern. In order to improve accuracy of judgement, it is necessary to compare this location with the band-like resist pattern at other peripheral vicinities.
Thus, when semiconductor circuits are being produced, in order that satisfactory resist patterns can be formed, it is necessary to periodically (and at other times as necessary) check a suitable range of the depth of focus, and provide feedback of results of this checking to a production line.
In particular, in continuous production, under relatively constant conditions in which resist patterns are repeatedly formed continuously with the same production conditions over a long period, the suitable range can be checked accurately and conveniently, but it is necessary to provide feedback of the results to the production line rapidly.
Furthermore, there are cases in which inconsistency factors in the production process relating to the formation of the resist pattern may become large due to changes in production conditions, adjustment and maintenance of a production line, unforeseen causes and the like. A primary demand is for the suitable range to be accurately determined without suffering the effects of such inconsistency factors of production.
In view of the above, the accuracy with which the conventional method finds the upper limit value is poor, and the conventional method has been unable to satisfactorily meet the needs of production.